Scale Size and Orientation of Structures at the Earth's Quasi-Parallel Bow Shock

Details Scale Size and Orientation of Structures at the Earth's Quasi-Parallel Bow Shock Scale Size and Orientation of Structures at the Earth's Quasi-Parallel Bow Shock

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmsh51b..08l&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #SH51B-08

Physics

2154 Planetary Bow Shocks

Scientific paper

Spacecraft observations of the quasi-parallel bow shock, where the magnetic field is approximately aligned with the shock normal, are characterised by an extended and disturbed transition. An integral part of the shock are perturbations to the upstream plasma characterised by large enhancements in the magnetic field magnitude and plasma density, and a reduction and deflection in flow velocity, called pulsations or SLAMS (short, large amplitude magnetic structures). We present four point Cluster measurements of SLAMS and compare their characteristics with those of the ultra-low frequency (ULF) waves found further upstream. The magnetic field signatures of SLAMS at spacecraft separated by about 100 km are well correlated, in contrast to the differences observed, especially in the magnetic field magnitude, at 600 km tetrahedron scales. We describe the appearance of the SLAMS, where the magnetic field magnitude increase is often preceded by a region of small scale variations which are uncorrelated between spacecraft, even at 100 km separation. Resolving differences between spacecraft pairs as a function of separation parallel and perpendicular to the plasma flow, we do not find evidence for time evolution in the short convection time of the SLAMS over the tetrahedron. Most SLAMS have the same form at all four spacecraft, but when differences occur they tend to be greater for larger flow-perpendicular separations. This is consistent with magnetic field changes at the edges of the structures occurring on scales comparable with the tetrahedron size. We compare SLAMS orientation and motion with those found for ULF waves. We find that the SLAMS and ULF waves have sunward directed velocities, as expected. The ULF wave properties remain relatively constant over several hours, but the apparent orientation of the ULF waves varies systematically within a single wave period. Comparison between ULF waves and SLAMS suggests that the latter have the same orientation as one part of each ULF wave cycle, consistent with the SLAMS preferentially developing from field variations during one phase of the ULF wave.

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